Gerri Detweiler, credit expert with Credit.com, said the Credit CARD Act, which amended the Truth in Lending Act and went into effect in Feb. 2010, aimed to make things more straightforward for consumers. One of the bank requirements of the CARD Act is to disclose the "conditions under which a finance charge may be imposed, including the time period (if any) within which any credit extended may be repaid without incurring a finance charge."

But Detweiler says, "Credit card interest calculations can get complicated, especially when you have different balances under different interest rates."

"So it's really important for consumers to pay attention to their statements, and when something doesn't look right to speak up," she said. "If you aren't sure whether your interest is being calculated correctly, you may want to reach out to the Consumer Financial Protection Bureau which may be able to help as well."

Tim Blood, Murr's attorney, said, "No company, and certainly not a large financial institution like Capital One, should promise its customers something, but then not follow through on its promise. Worse, here Capital One took far more money from its customers than if they had never done business with them. Through this lawsuit, Mrs. Murr intends to make Capital One stick with the deal it made and, hopefully, deter Capital One from running similar scams in the future."

For consumers, experts say the best way to avoid interest charges on promo offers is to pay the card down completely, take the transfer deal, and make no new charges on the card until it's paid off. It's a lesson that Murr learned the hard way.